The invention relates to optical smoke detectors, especially for use as early-warning automatic fire detectors.
Among the many types of commercially available automatic fire detectors, smoke detectors are particularly suitable as early-warning detectors, for timely fire-fighting intervention.
In the main, smoke detectors may be classified as ionization smoke detectors or optical smoke detectors. Response of the former is based on adsorption of atmospheric molecules on smoke particles; in the latter, optical properties of aerosols are used for smoke detection. This may involve sensing of attenuation or extinction of an optical beam by smoke ("extinction detector"), or sensing of optical scattering at smoke particles ("scattered-light detectors"). Since extinction by smoke is a relatively weak effect, a measurement distance has to be relatively long for positive smoke detection, or costly design and/or electronic measures are required for positive fire detection. In scattered-light detectors, the measurement distance can be relatively short, and these are most prevalent as so-called point detectors.
In German Patent Document DE-A1-2,822,547 (Hochiki Corp.; Dec. 7, 1978), a line extinction detector is described which includes a light source. After traversing a measurement distance, a portion of light emitted by the light source reaches a radiation receiver. If smoke is present in the measurement distance, the output signal of the radiation receiver will be reduced as a function of smoke density. This output signal is fed to a threshold-and-comparison circuit followed by an evaluation circuit for triggering an alarm signal if the output signal falls below a predetermined alarm-threshold value. Lenses are disposed in front of the radiation source and of the radiation receiver, for focusing of the light traversing the measurement path. The focusing systems are very costly.
Most of the older scattered-light smoke detectors are based on forward-scattered light, with large smoke aerosol particles having a strong effect, and with small smoke particles causing little light scattering. Smoke detectors based on back scattering have more uniform sensitivity and are more universally suitable. However, due to lower intensity of back-scattered light, more elaborate electronic circuitry is required. Moreover, as scattered light is likely to be reflected from the walls of a housing into the receiver, a complicated optical labyrinth or radiation trap is required (e.g., a plurality of diaphragms as disclosed in European Patent Document EP-A1-0,031,096) to keep internally reflected light from the radiation receiver.
In combination with optical focusing lenses in front of the light source and the receiver, the diaphragms in the measurement chamber of the smoke detector according to EP-A1-0,031,096 further serve for focusing of the light beam directed to the measurement volume, and of the radiation scattered from the measurement volume, for shortening of the smoke detector.
Size reduction is desirable not only for aesthetic reasons, but also for the sake of simplified mass production of smoke detectors. Accordingly, German Patent Document DE-A1-3,743,737 (Hochiki Corp.; Jul. 7, 1988) discloses a smoke detector shaped especially for compactness. However, economical mass production is impeded in that a separate, possibly manual assembly step is required to provide a circuit board with a wired photodiode.
Parasitic scattered light, e.g., as caused by contamination, can reach the optical receiver from the measurement chamber. German Patent Document DE-A1-3,831,654 (Beyersdorf; Mar. 22, 1990) discloses detection of contamination of the measurement chamber by means of a second photodiode, for preventing an alarm if contamination exceeds a predetermined value.
For size reduction, and to reduce the number of components, British Patent Document GB-A1-2,236,390 (Matsushita; Apr. 3, 1991) discloses a scattered-light smoke detector which includes a wired IRED radiation source in an integrated circuit on a printed-circuit board, and a radiation-receiver photodiode lying flat on the board. A prism with integrated lens serves as deflection and focusing element for concentrating the scattered radiation from the measurement chamber onto the photodiode. This prism with integrated lens is relatively costly; moreover, the required precise placement of the lens is quite complicated.
In European Patent Document EP-A1-0,462,642 (Ajax de Boer; Dec. 27, 1991), scattered-light smoke detectors are disclosed in which the polarization of scattered light is used for detecting smoke concentration, particle size and, up to a point, particle shape. These smoke detectors have more uniform response to different types of fire.
Optical smoke detectors using electronic components, for producing signals to an evaluation circuit for determining the presence of smoke are further shown and described, e.g., in U.S. Pat. No. 4,119,949, issued Oct. 10, 1978 to E. G. Lindgren and in U.S. Pat. No. 4,857,895, issued Aug. 15, 1989 to E. K. Kaprelian.
The cited patent documents include no teaching concerning a more compact design or simplified construction of optical smoke detectors.